This research project involves studying the ontogeny and regulation of the mammalian pituitary-adrenal system. The ovine fetus with chronically maintained catheters is the model used to study both basal and stimulated fetal pituitary-adrenal activity in a relatively undisturbed intra-uterine environment. Pituitary-adrenal function is also systematically investigated in neonatal lambs during the first month of life. Results of the in vivo experiments are combined with in vitro studies on pituitary-adrenal function to delineate possible mechanisms involved in the changes in activity which occur during gestation.